With increasing frequency, hollow bar micropiles are being used as a foundation
solution along Coastal South Carolina. On a recent project, micropiles were installed
using a size 40/20 and 52/26 continuously threaded, hollow core steel bars. The hollow
space in the bar allowed an innovative application of Thermal Integrity Profiling (TIP)
equipment, normally used for drilled shaft installation verification, to be applied to
micropiles. We utilized a single strand of thermal wire sensors inserted into the hollow
center bar after completion of micropile installation. A data logger recorded the concrete
temperature over a 24 to 36 hour period as heat was generated during the exothermic
hydration reaction. Processing software then used the temperatures recorded to generate
a 3-dimensional model of the micropile.
The adopted building code in South Carolina is the 2015 IBC, which requires full
time inspection of cast-in-place foundations and requires verification of element
diameters. Using the known pile length and grout volume, the average pile diameter can
be calculated. However, none of the required inspection methods allows direct verification
of the pile diameter, which may vary with depth and soil strata.
TIP testing can also detect micropile defects including cave-in or clogged soil
cuttings which prevent full grout cover and protection of the steel center bar from
corrosion. Even load tests, which are the ultimate proof of capacity, cannot guarantee
that defects are not present. While load tests are generally performed on 1% to 2% of the
production piles, the ease of TIP testing allows for inspection coverage of 10% to 20%.
While this method of verification cannot replace full time inspection of a qualified
inspector or load testing, it can be performed relatively easily, and is another valuable
and innovative method to determine proper pile installation.
Subjects: hollow bar micropiles, thermal integrity profiling, TIP testing
Author: William B. Wright, P.E., Matthew, L. Silveston, P.E., Thomas C. Smoak, III, P.E., William L. Snow, Jr., P.E.
Publisher: ISM, DFI, ADSC